Upgrades?

[VeryBigSword]

LOL that is my exact strategy - in my recently finished game as Inca/Huayna, the original Quechua from 4000BC was still fortified in Cuzco as the sole garrison when I won space race.

I do however like to upgrade City Raider III macemen and axemen to rifles or grenadiers and later infantry, making them top-notch offensive units during those eras. After all you can't build gunpowder CRs. Not so important once you get tanks though, so I don't bother upgrading to mech infantry.

[Asmodeous]

I only really upgrade when desperate or when upgrading the unit would give a huge advantage. (Like Upgrading Combat 1, Drill 4 Longbowmen into Riflemen...)

Otherwise I just rebuild and replace most of the time, if necessary. I also leave the interiors defended by crappy units, except for against humans that are smart enough to skip past the well-defended cities and go straight inland.

Me.

[lockstep]

I always try to upgrade macemen with city raider II or III to riflemen/infantry because "new" gunpowder units don't have access to the city raider promotion line.

[Crossfire]

If I get a great merchant I will often use him to get the wealth to do an upgrade of knights to cavalry. These can then be used for a major expansion because I usually get them a while before and AI does.

I often have lots of money in the late game too and upgrade anyone else that might come under attack.

[BigWilly1974]

If I have a somewhat significant army, usually with a good backbone of experienced units, I'll typically turn tech slider down to 0 for a few turns if necessary to do a mass upgrade just prior to launching a new war. I won't upgrade all troops, but any offensive troops that I can bump up a few strength points, I'll typically do this (swordsmen to mace; knights to cuirass, cuirass to cavalry). Beyond cavalry, I'm less likely to upgrade as by then I've generally got a strong industry and can churn out new units quickly.

[Yosho]

I find upgrading to be useful at certain times. Very often, if I'm the first one to a key millitary tech like assembly line, I and upgrade a bunch of rifles and muskets and whatever else I've got even down to macemen and stuff right away and then rush someone, I'll even turn my tech slider down for a few turns to get the cash needed to do all those upgrades. Otherwise, on marathon, I'm probably waiting 7 turns before my first infantry units pop out. If you upgrade right away, you can get a large infantry army much faster, and that gives you more time to bash heads before the AI catches up and also gets infantry, and even if it costs you 1500-2000 gold to get started a fast, efficent war is much, much cheaper in the long run then a slow war where war wiereness starts to build up.

[Supr49er]

I always upgrade my navy as soon as the new units are available. The aggressive AI can and will use large naval attacks. I always have a military city with HE, WP, Pentagon, and Military Academy to produce high XP units for attack. I will upgrade my defensive units on the border only. I agree that in SP the AI won't go for interior cities.

[Arrian]

5XP (or more) Macemen upgraded to Infantry is definitely worthwhile. City raider II or III infantry rock. Same thing for catapults/trebs -> cannon.

I don't tend to upgrade units that have less than 5XP, though I will in a pinch, of course. The exception, for me, is ships. I will upgrade trimeres to caravels to get a jump on circumnavigation, and caravels to frigates so I don't have to build frigates (or have to build fewer frigates). I'm bad about building enough naval units, so deleting them would be even worse.

-Arrian

[joncnunn]

I also noticed that seige unit upgrades actually seem to be reasonably cost.
Athough since I often end up using Trebs/ Cats as sucide units to soften down the opposing army after having bombarded the defenses to nothing, I don't tend to actually have many of them to upgrade.